In an earlier filed, co-pending application, U.S. application Ser. No. 582,200 filed May 30, 1975, in the name of the applicant herein, there is described an electrical load transfer control system whereby two power sources, one of which may be an inverter and the other of which may be a commercial power source, are arranged so that under normal operating conditions the electrical energy required by the load is provided to it from the inverter and the commercial power source is used as a standby in the event of failure of the inverter. In such a system, a ferroresonant circuit is in series with the inverter, and has a storage element with enough energy storage capacity so as to carry the load for a short period of time; that period of time being sufficient to enable a fast operating switch to close to connect the commercial power source to the line and then a slower operating switch may open so as to disconnect the already failed inverter from the load. However, such a system may, unless proper switches are provided having power isolation, preclude isolation of the otherwise disconnected commercial power line from the load, so that in certain circumstances where the commercial power line may not be operating, power may be fed back to the commercial power line unless, as stated in the aforementioned co-pending application, proper switching arrangements are provided so as to assure complete power isolation of the commercial power line.
The ferroresonant voltage regulating circuit having an energy storage element which is provided in the inverter -- and, as will be discussed hereafter with respect to the present invention, also in series with the commercial power source -- is advantageously one such as that taught in applicant's U.S. Pat. No. 3,824,449 issued July 16, 1974.
It is occasionally a requisite of AC power systems to provide such a system whereby transfer switching systems may not be desired but where inherent fail-safe, regulated uninterrupted no-break power may be supplied to a critical load, regardless of failure either of a commercial power source or of an inverter of an uninterrupted power supply (UPS) system. This may be especially true if the load is one which is not always constant, because a parallel operated system having a conditioned or regulated AC line in parallel with an inverter, each supplying approximately one half of the total load power, is such that it can supply four times the in-rush current of a single non-redundant inverter supplying the same load.
It is also possible that requirements may arise for a higher regulated load current to be delivered to a load than may be available from one commercial source, but at the same time a second commercial source of a different voltage -- or indeed, frequency -- may be available whereby the load power requirements might be shared between the two commercial sources. In that case, for purposes of stability and assurance of UPS system operation, float chargers and standby batteries may be provided together with an inverter on one of the lines, with the output of the inverter being synchronized to the other commercial line with which it would then be operated in parallel, in accordance with the principles of this invention.
Thus, this invention provides a line redundant power system having an inverter in synchronism with and in parallel with a commercial power source, each sharing a load, and having means to assure isolation of the commercial power line in the event of failure thereof so as to preclude negative power flow from the inverter to the commercial power line.
It is important to note that failure of a normal operating inverter having sufficient standby battery energy and float charging circuits therefor -- so as to assure operation of the inverter in the event of failure of the commercial line supplying the normal operating power for the converter which is rectified and also used to maintain the charge level of the batteries -- is statistically much less likely to happen than failure of the commercial power source or operation of that power source outside of predetermined limits, may happen. Thus, in a power system which is line redundant -- having the commercial power line operated in parallel and load sharing relationship with an inverter -- it is important to provide means to isolate the commercial power line in the event of its failure. Inverters would, in any event, carry normal isolating circuitry; and the inverters may be of the sort taught in applicant's U.S. Pat. No. 3,931,565 issued Jan. 6, 1976.
It has often been the case, in the past, where an AC power system was provided having a DC driven standby power supply. Such a system is taught in O'Sullivan U.S. Pat. No. 3,351,770, issued Nov. 7, 1967. That patent, however, teaches a standby system whereby a normal AC line is operated in parallel with an inverter, but that the AC input provided by the normal power line is at a somewhat higher voltage than the inverter output so that most of the power supply to the load is from the AC line. In the event that the AC line fails, either short or open, or drops slowly, the inverter is arranged to deliver the necessary proportion of the power to maintain the load, and in addition the power required by the total impedance of the AC line. This results, however, in a totally unsafe system and one which would be completely unacceptable to utilities and hydro power authorities and commissions in most jurisdictions, because the system permits for energy interchange between the line and the inverter passed the load, depending on the line and load conditions. In particular, the O'Sullivan patent provides for energy interchange from the inverter to the commercial AC line, an unacceptable circumstance.
Thus, the present invention provides a system where normally the inverter and the commercial power line -- which has a conditioned, regulated output -- share the load approximately equally; but where, in the event of inverter system failure, the conditioned and regulated commercial power line supplies the full load, and in the event of failure of the commercial power source, the UPS system inverter supplies the full load. The present invention provides a parallel redundant the load sharing regulated AC system whereby any reverse power flow in a direction from the load towards the commercial power source is blocked, sensed and isolation of the commercial power source is initiated.
Because the operating systems -- the commercial power source and the UPS inverter -- are sharing the load, and because each has an energy storage element included in it as part of the voltage regulation -- the ferroresonant circuit which is normally used being, in any event, downstream towards the load from the commercial power source or the inverter, respectively -- there is no undue power loss or interruption of power being fed to the load in the event of failure of either side.
It is important to note that the present invention may also relate to parallel line-regulated systems where two conditioned and regulated commercial AC sources which are operated in synchronism but which are otherwise independent one of the other are operated in parallel. In such circumstances, of course, appropriate logic circuits are required for sensing the synchronism between the two independent commercial AC sources, as well as for other supervisory and sensing purposes.